Image based updates

With Ubuntu getting ported to more and more mobile devices, the standard update mechanism through apt and dpkg is starting to show some limitations.

As a result, Ubuntu is now looking at supporting an additional way of updating devices that are based on read-only images. For those devices, we're looking at doing image based updates, basically generating new images on the server side with the usual tools (debootstrap, apt, dpkg), then generate deltas between those images and have the client pull the delta and apply it without ever actually dealing with packages.

We don't intend this to become the only way to update Ubuntu machines, and the use of apt/dpkg will remain supported with most devices having some kind of switch to opt out of image based updates and re-enable apt.

Work items for ubuntu-13.07:
[barry] LP: #1192585 Implementation of the client DBUS API (to be used by the updater UI): DONE
[gema] Figure out if there is any image system testing to do this cycle for this work: INPROGRESS

Work items for ubuntu-13.08:
Implementation of the updater UI: TODO
[barry] end-to-end testing through lxc containers: POSTPONED
[mpt] Design of the updater UI for system updates: TODO
[sergiusens] flashing from cdimage update zips to this system should preserve as much data as possible: INPROGRESS

Work items:
[stgraber] Look into dpkg's support of multiple status files (or multiple /var/lib/dpkg directories actually): TODO
[vorlon] investigate unionfs options with the constrained requirements for a system like this: TODO